Is there another backup on the server? If so, try removing it first. Even if your quota is bigger than the total size of your site and all of your backup archives - or if you have unlimited quota - you are bound by the real available disk space of the physical hard disk your account is hosted on.
This is a known limitation of all shared services. For example, a host may put 200 sites with 1Gb limit on a 130Gb disk, on the premise that most sites will consume far less than their quota. If all 200 sites are at 647Mb, the first site which tries to occupy up to 1Gb space will face a problem: the disk runs out of disk space. When you're talking about backups in the range of 30Gb, this can happen very easily. A site of that size and two backups occupy ~90Gb. Given that most hosting-grade hard disks are in the capacity range of 100-150Gb (which must also contain the operating system) it's too easy to start running out of disk space. Maybe you want to check this speculation of mine with your host.
Nicholas K. Dionysopoulos
Lead Developer and Director
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